Method for the reduction of nickel from an aqueous solution

ABSTRACT

The invention relates to a method for the precipitation of nickel from an aqueous solution containing its sulphate as a metallic powder suitable as an alloying element for refined steel. In this method, nickel reduction takes place continuously in one or several autoclaves at a temperature of 80-180° C. and hydrogen pressure of 1-20 bar, whereby the production capacity can be raised significantly, compared to batch processes made in correspondingly dimensioned devices or equipment.

The present invention relates to a method for the precipitation ofnickel as a metallic powder suitable for the production of refined steelfrom an aqueous solution containing its sulphate. In this method, nickelreduction takes place continuously in one or several autoclaves at atemperature of 80-180° C. and hydrogen pressure of 1-20 bar, whereby theproduction capacity can be raised significantly, compared to batchprocesses made in correspondingly dimensioned devices or equipment.

The production of nickel from an aqueous solution by hydrogen reductionin autoclaves as batches has been in use on industrial scale since the1950s. The method is described in articles such as: Benson, B., Colvin,N.: “Plant Practice in the Production of Nickel by Hydrogen Reduction”,pp. 735-752 in the conference publication: Wadsworth, M. E., Davies, F.T. (ed.): “Unit processes in Hydrometallurgy”, International Symposiumin Hydro-metallurgy, Dallas, Feb. 24-28, 1963, Gordon and Breach, NewYork, 1964. The production method described in the article is still inuse through-out the industry and according to the article the methodbased on the batch principle comprises the following stages: nucleusreduction, reduction and leaching.

In the batch process, nickel nuclei are made in an autoclave by hydrogenreduction using an FeSO₄ catalyst. When the nuclei are ready, the mixersare stopped, the nuclei are allowed to settle and the solution on top ofthe nuclei powder is blown off. In the reduction stage the actualprocess solution is fed into the autoclave and metallic nickel isreduced from this with hydrogen on top of the nuclei. Reductiontypically occurs at temperatures of 199-204° C., and at overpressures of24-31 bar. When reduction has ended, the mixers are stopped, the powderis allowed to settle to the bottom of the autoclave and the solution isremoved from on top of the settled powder. The method is repeated 50-60times and some nickel powder is also removed when the solution isremoved. The reduction series or cycle is finished when the particlesize of the nickel powder grows so large that its suspension in theautoclave becomes difficult or when the reduction time of one batchbecomes too great. At the end of the reduction cycle the whole autoclaveis emptied. Any metallic nickel stuck to the inner structure of theautoclave is dissolved off between cycles.

It is clear to a person skilled in the art that the actual reductionstage of the batch process comprises at least the pumping of thepre-heated solution to the autoclave, the hydrogen reduction of thebatch of the nickel solution, the settling of the nickel powder andblowing off the residual solution from the top of the nickel powder. Allthese sub-stages are performed as consecutive actions, notsimultaneously. However, only the hydrogen reduction of the nickelsolution is effective time from a production point of view and it can becalculated from the above-mentioned article by Benson and Colvin, thatthis operation uses only 45% of the total time. The capacity of themethod can be calculated from this article as:

251 batches×46 g Ni/l/(14d* 24h/d)=approx.34 (g Ni/l)/h.

The article by Evans, D. J. I.: “Production of Metals by GaseousReduction from Solution”, Processes and Chemistry, Paper 35/Advances inExtractive Metallurgy, A symposium in London, April, 17-20 , 1967, TheInstitution of Mining and Metallurgy, mentions that the particle sizegenerated by the nuclei reduction described above is of the order of0.001 mm.

Metallic nickel production by hydrogen reduction as a continuous processis presented in U.S. Pat. No. 2,753,257. The patent mainly describesreduction in batch processes, but the examples also mention continuousprocesses. In relation to continuous processes it is stated that amaximum yield of 80% can be achieved and that the batch method should beused for better results. It is characteristic of the said method firstlythat the composition of the solution is adjusted twice, and secondlythat the iron present in the solution has an adverse effect on thefunctioning of the method.

In U.S. Pat. No. 2,753,257 the composition of the solution is firstadjusted to the optimum demanded for self-nucleation. In the secondstage the composition of the solution is adjusted so that it is optimalfor the reduction of the metal powder on top of the metal nuclei. It isalso supposed in the method that iron is eliminated from the solution bysome known method to content levels that do not interfere with thereduction of the metallic powder. The method is performed at atemperature range of 218-232 ° C. and at 52-55 bar of overpressure.

Another continuous process is presented in U.S. Pat. No. 3,833,351. Thisdescribes a method for the production of copper, nickel, cobalt, silveror gold powders from a solution prepared by acid or ammoniacal leaching.Powder production is carried out by reduction with hydrogen gas in acontinuous vertical tubular reactor, where the height to diameter ratioof the reactor is at least 10:1. In the patent description it statesthat powders can be produced in the reactor even in atmosphericconditions. However, the section describing the production of nickel forexample reveals that if reduction is carried out in conditions where theaverage temperature of the reactor is 93° C. and the pressure about 32bar (Table III, Run 2), the resultant solid matter contains only 55%nickel. If economically viable results are required, reduction must becarried out in conditions where the total pressure is for instance inthe range of 33 bar and the average temperature 140° C. with a maximumtemperature of 225° C. (Run 1), whereby the amount of nickel powderformed is 90% of the solid matter. The resulting nickel is not onlyimpure but also extremely fine and thus awkward to handle. The size ofthe powder produced was 0.001-0.002 mm for copper and so small fornickel and cobalt that ordinary settling and filtering may no longerwork, requiring perhaps even magnetic separation in order to separatethe particles from the solution. The fineness of the powder also greatlyhinders washing. This method has never been implemented on industrialscale.

Autoclaves equipped with partitions are used for continuousprecipitation and leaching autoclaves, as described e.g. in the articleby F. Habashi, Pressure Hydrometallurgy: Key to Better and NonpollutingProcesses, Engineering and Mining Journal, February , 1971, pp. 96-100and May 1971, pp. 88-94. Partition walls have not been used in reductionautoclaves.

From the above, we can conclude that the nickel hydrogen reductionmethod has worked relatively well in the batch process, and attempts toconvert to a continuous process have been rather poor. The reasons forthis have probably been the high temperatures and pressure used inreduction processes, which have made it difficult to change the processover to a continuous one.

A continuous process is cheaper than a batch process, because theproduction capacity of equipment of the same size is greater than thatof a batch process. Now, with the method of the present invention,nickel powder especially suitable as an alloying element for refinedsteel can be produced by performing continuous hydrogen reduction of anickel sulphate-containing aqueous solution in a pressurised space ineasier conditions than earlier, wherein the hydrogen pressure is in therange of 1-20 bar and the temperature in the range of 80-180° C.,(preferably with hydrogen pressure from 2-10 bar and the temperaturefrom 110-160° C.). According to the invention, at least one autoclave isused as the pressurised space, being equipped with partition walls,which divide it into several sections with mixers, or severalconsecutive autoclaves with mixers, which autoclaves may be single ormulti-sectioned. The invention is particularly advantageous when usingnickel sulphate solutions obtained in acid leaching and which thereforedo not practically contain ammonium sulphate. The essential features ofthe invention will be made apparent in the patent claims.

Nickel sulphate-containing aqueous solutions are generally prepared byleaching either nickel concentrate such as laterites orpyrometallurgically produced nickel mattes. The leaching may be eitheracid or ammoniacal. The nickel content of the sulphate solution usuallyremains lower in concentrate leaching than in matte leaching, but ifliquid-liquid leaching is used as one solution purification step, thenickel content can easily rise to over 100 g/l. In the framework of thisinvention, reduction is performed from a solution with a nickel contentof minimum 30 g/l, preferably at least 50 g/l and most advantageouslyminimum 80 g/l.

For reduction in a pressurised space, the composition of the nickelsulphate solution feed is adjusted before reduction in a preparationstage, which comprises a number of mixing reactors. The adjustment ofthe solution composition is carried out only once. If there is any ironin the solution, ferrous sulphate is made use of to form nuclei, onwhich nickel powder is reduced. If the amount of iron in the solution isnot sufficient as it is, iron is added to the solution. In place of ironor in addition to it, chrome can be used for nucleus formation as chrome(II) sulphate CrSO₄. Ammonia can also be used for composition adjustmentas can the feed of other additives and admixtures normally used inreduction.

If autoclaves divided into sections are used in embodiments of theinvention, the upper edges of the partitions are essentially horizontaland their heights from the lowest point of the bottom of the autoclaveis graded so that the height of the partition walls seen in thedirection of the solution flow decreases, so that the surface of thesolution in the sections decreases correspondingly. Gradation can ofcourse be implemented in some other suitable way, for example, so thatthe partitions are the same height, but have discharge slots orapertures at different heights. The purpose of the partitions is toimprove the efficiency of the autoclave.

The method of the invention is described further by the accompanyingdrawings, where

FIG. 1 is a vertical section of the principles of an autoclave of theprior art and

FIG. 2 a vertical section of the principles of an autoclave divided intosections with partitions according to the invention.

FIG. 1 is an example of a reduction autoclave 1 of the prior art,functioning on a batch basis, which autoclave is single-sectioned andequipped with a feed and discharge pipe 2 for the slurry to be reduced,mixers 3, gas feed pipe 4 and gas exhaust pipe 5. The number of mixersin the autoclave can be changed as can the positions of the slurry andgas feed points.

Reduction autoclaves of the prior art do not have partitions dividingthe space into sections—the whole pressurised space is integrated.

When the method according to the present invention is implemented in asingle autoclave, it is preferable to use an autoclave as in FIG. 2,which is in principle the same type as presented above, but equippedwith partitions 6 and a discharge pipe 7 for the solution and the solidmaterial at the back section of the autoclave. The autoclave shown inthe figure is a typical horizontal cylindrical shape. When thesuspension of solution and solid matter have been discharged from theautoclave, the nickel powder is separated from the end solution bywell-known methods such as filtration. As stated above, the heights ofpartitions 6 are graded from the bottom 8 of the autoclave so that theheight of the partition walls decreases in the direction of the solutionflow. The number of mixers and sections 9 is not restricted to the fourshown in the diagram, but can be changed. Preferably there will be 3-6sections divided by partitions, but that can also vary if the needarises. The mixers may be single or multi-bladed. It is clear to askilled person that the partitions may include apertures and otherstandard components to improve the efficiency and ease operation of theautoclave at various points, in the normal way.

An autoclave according to this invention may also be an integral type asin FIG. 1, wherein several of them are positioned one after the other ina series in continuous methods. In this case the single autoclave isequipped with a separate discharge pipe 7 for removing the solution asin FIG. 2, through which the solution is conveyed to the next autoclave.A combination of said autoclaves may also be used i.e. there may besingle-section and multi-section autoclaves connected consecutively in aseries. A single-section autoclave may also be for example a verticalcylindrical form, but single-section autoclaves are also always equippedwith mixers.

When hydrogen reduction of an aqueous nickel solution (nickel sulphatesolution) is performed with the method now developed, significantlylower temperatures and pressures can be used in the autoclave than shownin the prior art. Thanks to this, the hydrogen reduction of a nickelsolution can be converted or be made continuous from the start, wherebythe capacity of the autoclave or the group of autoclaves risesconsiderably compared to the batch process. Thus the nickel solutionhydrogen reduction process can be operated continuously, when thehydrogen pressure is in the range of 1-20 bar and the temperature from80-180° C., preferably at a temperature of 110-160° C. with the hydrogenpressure in the range of 2-10 bar.

For example Fe²⁺and Cr²⁺are used as reduction catalysts, which are addedto the reduction feed solution at the feed solution preparation stage,just before the solution is charged into the autoclave or directly intothe reduction autoclave. The catalyst is charged at least partially insolution form. Fe²⁺and Cr²⁺as catalysts are not harmful to the qualityof the product. Two thirds of the nickel produced in the world iscurrently used in the production of refined steel. Consequently, anyiron contained in the nickel is of no concern. Should chrome be used asreduction catalyst instead of iron, traces of the former will not causeany problems in refined steel production either. Iron and chromecompounds are preferably iron (II) and chrome (II) sulphates, but it isalso possible to use other such chemical compounds as catalysts that donot harm refined steel production or that are removed from the nickelpowder during the sintering of briquettes.

Where necessary well-known additives are used with the purpose ofpreventing the plating of metal on the autoclave walls and other innerelements, and/or of influencing the form of the powder flakes or theiragglomeration or dispersion tendencies.

The acid generated in reduction is neutralised preferably with ammonia.Advantageously ammonia is mixed into the solution at the preparationstage before the solution is fed into the autoclave, but an ammoniaaddition may also be made directly into the autoclave. In both cases, itis beneficial to select the amount of ammonia so that the mole ratioNH_(3/)Ni (=added ammonia/total amount of nickel in feed) is 1.6-2.4.

If the metallic nickel generated in the autoclave includeshydroxide-containing compounds, they can be leached off in the methoddescribed in U.S. Pat. No. 3,833,351 with an ammonium hydroxide and/orsulphuric acid solution and the solution obtained returned to a part ofthe process prior to reduction i.e. the solution preparation stage,preferably to its final reactor.

When the capacity of the present method is calculated in the same way asfor embodiments of the prior art, a result of 100-130 (g Ni/l) Th isobtained. The capacity achieved with the method of the present inventionis thus at least twice that of the method described in the prior art.With the present developed method nickel powder suitable as raw materialfor the refined steel industry as such, in briquette form or inbriquettes and sintered, can now be produced at an amazingly largecapacity, at an amazingly low temperature and pressure.

After reduction occurring in a pressurised space there is always alittle nickel left in the solution i.e. in the end solution to beremoved after separation of the nickel powder. The method now developedallows for variation in nickel content in the end solution. If thisso-called residual nickel amount is very small, e.g. under 1 g/l, it maybe recovered by for instance sulphide precipitation or ion exchange, andreturned to a stage in the process preceding reduction. If the amount ofresidual nickel is greater,it can also be crystallised from the endsolution in a known way such as by cooling, evaporation and if necessaryusing ammonium sulphate additive as nickel ammonium sulphate. The smallamount of nickel left in the solution after crystallisation can beremoved, for example, by sulphide precipitation or ion exchange.

If the nickel content of the remaining solution is fairly small, theresulting nickel ammonium sulphate NiSO₄.(NH₄)₂SO₄.6H₂O can be dissolvedwith an addition of ammonia into the feed solution at the autoclave feedsolution preparation stage, whereby the nickel cycle in the process ismade as short as possible.

If the nickel content of the residual solution is so great thatreturning NiSO₄.(NH₄)₂SO₄.6H₂O to the nickel autoclave reduction wouldraise the ammonium sulphate content of the reduction feed solution somuch that the reduction of nickel would be slowed down significantly,the NiSO₄.(NH₄)₂SO₄.6H₂O can be dissolved with ammonia into a solutioncontaining ammonium sulphate and the solution thus obtained fed furtheras in the method described in the Benson and Colvin article into aseparate autoclave operating on the batch principle.

The invention is illustrated in more detail by the following examples:

EXAMPLE 1

A test was made in a horizontal cylindrical autoclave, which was dividedinto six sections by partitions. In addition to the sections thepartitions further divided the autoclave into two spaces: the gas spaceabove the upper edges of the partitions and the solution or slurry spacearound the partitions. The total volume of the autoclave was 75 l, ofwhich the gas volume was about a third and the slurry volume about 50 l.

The upper edges of the section partitions were essentially horizontaland their heights from the bottom graded so that the height of thepartitions dropped in the direction of the slurry flow. Thus the highestpartition was-in the feed section of the autoclave and the lowestbetween the last two sections. Along with the partitions the slurrysurface level also decreased: towards the back section of the autoclave.Owing to this the slurry fed into the first section flowed from onesection to the next by the effect of gravity ending finally in the lastsection, from where the slurry was removed from the autoclave by meansof the prevailing gas pressure in the autoclave.

Each section was equipped with an effective rotating mixer with abasically vertical shaft with two mixing elements on the same shaft asshown in FIG. 2. The mixers sucked the hydrogen gas from the gas spaceand dispersed it into the slurry, thus speeding up the dissolving of thehydrogen and the forming of nickel. The mixers also kept the nickelgenerated in the autoclave well suspended, which helped it to proceedfrom one section to another.

The ammonium sulphate free solution used in the tests had been throughsolution purification and contained on average 108 g Ni/l as sulphate.Gaseous ammonia was added to this as neutralising agent so that the moleratio was 2.2 mole NH_(3/)mole Ni and ferrous sulphate in aqueoussolution in order to form the nuclei so that the weight ratio became0.007 g Fe²⁺/1 g Ni. The addition of ammonia took place as a continuousprocess in several mixing reactors operating in series and at normalpressure. The slurry generated was pumped continuously into theautoclave so that the average retention time in the autoclave was 0.9 h.The addition of ferrous sulphate was made just before the solution wasfed into the autoclave i.e. into the feed pipe between the last mixingreactor and the autoclave.

The temperature of the mixing reactors was 80° C. and the temperature ofthe autoclave was about 120° C. and the hydrogen pressure 5 bar. Thetest lasted 56 hours, during which time an average of 5.3 kg Ni/h wasfed into the autoclave as solution and as precipitate. The end solutionto be removed from the autoclave after nickel separation contained onaverage 4.6 g Ni/l, in other words 0.25 kg Ni/h and the iron content ofthe solution was 0.11 g/l. The yield of nickel to metal was thus about95% and the calculated production capacity of the autoclave regardingslurry volume about 100 (g Ni/l)/h.

EXAMPLE 2

The autoclave described above was used in the test and the ammoniumsulphate free nickel sulphate solution used as feed had been throughsolution purification and contained on average 113 g Ni/l. Gaseousammonia was added to this so that the mole ratio was 2.0 mole NH₃/moleNi and ferrous sulphate was added so that the weight ratio became 0.007g Fe₂₊/1 g Ni. The addition of ammonia and ferrous sulphate to ok placeas in example 1. The average e retention time of the slurry in theautoclave was 0.8 h.

The temperature of the mixing reactors was 80° C. and the temperature ofthe autoclave was about 120° C. and the hydrogen pressure 5 bar. Thetest lasted 78 hours, during which time an average of 6.7 kg Ni/h wasfed into the autoclave as solution and as precipitate. The end solutionto be removed from the autoclave after nickel separation contained onaverage 2.2 g Ni/l, in other words 0.14 kg Ni/h and the iron content ofthe solution was 0. 17 g/l. The yield of nickel to metal was thus about98% and the calculated production capacity of the autoclave regardingslurry volume about 130 (g Ni/l)/h.

As stated above, the production capacities achieved in the examples areconsiderably higher than the capacities apparent from the articlesmentioned in the prior art. The examples presented above include alarger campaign, where nickel powder was produced with an iron contentof 0.1-2.0% and analysis otherwise corresponding to LME classification.According to sieve analyses of the powders, their 50% passing throughgrain size was about 0.050 mm, i.e. extremely large in comparison withthat in the above-mentioned method of the prior art-powder produced byso-called nuclear reduction, where the grain size is of the order of0.001 mm. The grain size is also larger than the powder produced by themethod in U.S. Pat. No. 3,833,351, where at most it (copper powder) wasof the order of 0.002 mm.

The powders produced in the examples were pressed and sintered intobriquettes, which after sintering in a hydrogen atmosphere contained0.64-0.91% Fe, about 0.01% S and about 0.02% C. and which had acompression strength of over 3000 kg/cm². The product is suitable foruse in the refined steel industry.

EXAMPLE 3

The following pair of tests illustrate the effect of the ammoniumsulphate content of the reduction autoclave feed solution on the nickelreduction rate. The tests were made in a three-sectioned continuous,FIG. 2 type autoclave, with the following operating conditions:temperature 120° C. and hydrogen pressure 5 bar. The feed slurries wereprepared as in example 2 and were pumped continuously to the autoclaveso that the retention time was 70 min i.e. about 23 min/section. In test3.1 the slurry did not contain any ammonium sulphate, but in test 3.2the amount was 34 g/l. The results obtained were as follows:

TABLE 1 (NH₄)₂SO₄ Ni content of solution content in g/l Test feed g/lSection 1 Section 2 Section 3 3.1 0 27.5 12.6 2.7 3.2 34 34.1 21.3 16.0

The table shows the slowing effect of ammonium sulphate on the reductionrate of nickel sulphate. It also shows that the nickel content of theend solution of test 3.2, that is the solution coming from the thirdsection, was 16.0 g/l and the ammonium sulphate content of the feedsolution 34 g/l, which as moles are approximately of equal size. In factin the case of test 3.2 the crystallised NiSO₄.(NH₄)₂SO₄.6H₂O from thereduction end solution can be returned to the reduction autoclavewithout essentially altering its operation.

EXAMPLE 4

The following series of tests illustrate the effect of temperature andpressure on the reduction rate. The series of tests was made in the samecontinuous, 6-sectioned autoclave as the tests in examples 1 and 2. Thetest conditions and results were as follows:

TABLE 2 Feed Conditions Product Ni NH₃/Ni Fe T p_(H2) Ni Test g/lmole/mole g/l ° C. bar g/l 4.1 100 2.3 0.5 120 5.0 15 4.2 100 2.3 0.5130 5.0 7 4.3 100 2.3 0.5 130 2.5 22

The feed did not contain ammonium sulphate and product refers to thenickel powder from the autoclave end solution after separation. Theamount of feed was 50 l/h or a total retention time of 1 h.

These results show that a relatively small change in temperature andhydrogen pressure can have a considerable effect on nickel reductionrate and on the nickel content of the autoclave end solution.

EXAMPLE 5

The crystallisation of NiSO₄.(NH)₄SO₄.6H₂O from the reduction endsolution was also tested. The test was made in a small laboratory mixingreactor. The feed solution, which contained about 5 g/l nickel and about100 g/l ammonium sulphate, was taken from the campaign of examples 1 and2.

In the crystallisation test, solid ammonium sulphate was added to thefeed solution so that the solution content was 380 g/l (NH₄)₂SO₄. Afterthis the pH of the solution was adjusted to a value of 3 and itstemperature to 40° C., when it was mixed in the reactor for 60 min.During mixing, the analyses of the samples taken from the reactor wereas follows:

TABLE 3 Ni content of Mixing time solution min g/l 0 4.2 10 0.89 30 0.6360 0.78

This shows that the residual nickel in the reduction end solution caneasily be crystallised to a level where the final nickel can be removed,for instance by sulphide precipitation or ion exchange.

What is claimed is:
 1. A method of the reduction of nickel powdersuitable as a component of refined steel, comprising reducing an aqueoussolution containing nickel sulphate in a pressurised space usinghydrogen as the reducing agent, the reduction occurring continuously ata temperature between 110-160° C. and at a hydrogen pressure between2-10 bar in at least one autoclave, which is divided into sections bypartitions, where each section is equipped with a mixer, and removingthe aqueous solution from the at least one autoclave.
 2. A methodaccording to claim 1, wherein the height of the solution surfacedecreases by section in the direction of the solution flow.
 3. A methodof the reduction of nickel powder suitable as a component of refinedsteel, comprising reducing an aqueous solution containing nickelsulphate in a pressurised space using hydrogen as the reducing agent,the reduction occurring continuously at a temperature between 110-160°C. and at a hydrogen pressure between 2-10 bar in a plurality ofautoclaves, wherein reduction occurs in said autoclaves, which arearranged in series and equipped with mixers, and removing the aqueoussolution from said autoclaves.
 4. A method according to claim 3, whereinthe autoclaves are single-sectioned.
 5. A method according to claim 3wherein the autoclaves arranged in series are both single andmulti-sectioned.
 6. A method according to claims 1 or 3, wherein theautoclaves are essentially cylindrical in shape.
 7. A method accordingto claim 3 wherein the aqueous solution is removed from said autoclavesthrough a discharge pipe.
 8. A method according to claim 3, wherein thenickel sulphate solution is obtained in acid leaching.
 9. A methodaccording to claim 1, wherein the nickel content of the aqueous solutioncontaining nickel sulphate fed into the pressurised space is at least 30g/l.
 10. A method according to claim 9, wherein the nickel content ofthe aqueous solution of nickel fed into the pressurised space is atleast 50 g/l.
 11. A method according to claim 1, wherein the compositionof the aqueous solution containing nickel sulphate fed into thepressurised space is adjusted at the feed solution preparation stage.12. A method according to claim 1, wherein a reduction catalyst is usedto aid reduction.
 13. A method according to claim 12, wherein iron (II)sulphate, FeSO₄, is used as reduction catalyst.
 14. A method accordingto claim 12, wherein chrome (II) sulphate, CrSO₄, is used as reductioncatalyst.
 15. A method according to claims 11 or 12, wherein thereduction catalyst is added to the feed solution at the preparationstage.
 16. A method according to claim 12, wherein the reductioncatalyst is added to the feed solution just before the solution is fedinto the pressurised space.
 17. A method according to claim 12, whereinthe reduction catalyst is fed directly into the pressurised space.
 18. Amethod according to claim 1, wherein the solution to be fed into thepressurised space is neutralised at the preparation stage with ammoniaso that the mole ratio becomes 1.6-2.4.
 19. A method according to claim1, wherein the nickel solution is neutralised with ammonia in thepressurised space so that the mole ratio becomes 1.6-2.4.
 20. A methodaccording to claim 1, wherein the nickel solution contains practicallyno ammonium sulphate.
 21. A method according to claim 1, wherein thesuspension of nickel powder and solution is removed from the pressurisedspace and from which suspension the nickel powder is separated.
 22. Amethod according to claim 21, wherein the nickel remaining in the endsolution after separation is removed by sulphide precipitation or ionexchange.
 23. A method according to claim 21, wherein at least part ofthe nickel remaining in the end solution after separation is removed asa binary salt NiSO₄*(NH₄)₂SO₄*6H₂O.
 24. A method according to claim 23,wherein when the majority of the nickel from the end solution has beenrecovered as a binary salt, the residual nickel is removed from the endsolution either by sulphide precipitation or ion exchange.
 25. A methodaccording to claim 23, wherein binary salt NiSO₄*(NH₄)₂SO₄*6H2O isdissolved in the preparation stage of the feed solution and returned asfeed for the continuous hydrogen reduction of nickel in a pressurisedspace.
 26. A method according to claim 23, wherein binary saltNiSO₄*(NH₄)₂SO₄*6H₂O is dissolved in the preparation stage of the feedsolution and fed to the hydrogen reduction of nickel as a batch process.27. A method according to claims 25 or 26, wherein binary saltNiSO₄*(NH₄)₂SO₄*6H₂O is dissolved using ammonia.
 28. A method accordingto claim 1 wherein the aqueous solution is removed from the at least oneautoclave through a discharge pipe.
 29. A method according to claim 1,wherein the nickel sulphate solution is obtained in acid leaching.